1. Field of the Invention
This invention relates to a glass article having a viewing light path which passes through at least one surface region over which the glass is matted whereby specular reflection from the matted region is reduced.
2. Background of the Related Art
It is well known that light is reflected at a boundary between two media with different refractive indices. If the media are thick enough that interference effects can be ignored, the amount of normally incident light which will be reflected will be approximately [(n1-n2)/(n1+n2)]2, where n1 and n2 are respectively the refractive indices of the two media. In the case of a glass/air interface, we can write n1.perspectiveto.1.5 and n2.perspectiveto.1, and the expression thus indicates that about 4% of light incident on a glass/air interface will be reflected at that interface, and about 8% will be reflected from the two surfaces of a glass sheet in air.
This reflection is a disadvantage for many purposes. As examples may be cited: instrument dial glasses, for example watch glasses; glass panels covering light emitting diode displays, and more particularly those covering liquid crystal displays; cathode ray tube screens, for example television screens and computer monitor screens; and sheets of glass held in picture frames to protect a photograph, painting or drawing.
Since it is the image of the light source on the glass which gives rise to problems in viewing through the glass, it has been proposed not so much to reduce the total amount of light reflected, as to reduce the proportion of reflected light which is reflected specularly. As an example, it is well known to etch the surface of glass so that its surface is matted, in that it is densely populated with pits which act to scatter surface-reflected light. As a result, a high proportion of the reflected light is reflected diffusely rather than specularly. An example of a method of producing such glass is set forth in Glaverbel's British Patent Specification No. 1,151,931.
Such surface matting also affects light transmitted by the glass, and a proportion of the light transmitted through such a matted region will also be diffuse. This can be an important disadvantage if it is desired to view an object or image located some distance behind the matted glass surface, since the diffusely transmitted light will make optical resolution of the object or image difficult or impossible. For example it will be appreciated that in television cathode ray tubes, especially in the larger sizes which are commercially available, the front glass may often be several centimetres thick, or a separate phosphor screen may be located several centimetres behind the front glass of the tube, and any matting treatment performed on the exterior surface of the screen will accordingly impair resolution of the image formed in the phosphorescent coating on the interior face of the screen.